Method and apparatus for preventing ion deposition on corrosion protection electrodes



April 8, 1969 w. P. BANKS ETAL 3,437,572

METHOD AND APPARATUS FOR PREVENTING ION DEPOSITION ON CORROSION PROTECTION ELECTRODES Filed 0612. 12, 1964 Cawreol. L :12

TiE-l Pawae INVENTOR. WLZ/AM BAA/A45 JOHN 0. 84/0800) A TTOQNE) Int. Cl. C23f 13/00 US. Cl. 204-147 6 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for applydng anodic protection to an electroplating system wherein a shield of such a nature as to prevent the passage of metal cations but at the same time permit sufiicient electrolytic communication between a cathode and the metal vessel to anodically protect said vessel is inserted around said cathode.

This invention relates generally to a shield for an electrode being used to minimize corrosion of a vessel containing a corrosive electrolytic solution. More particularly, but not by way of limitation, the present invention relates to apparatus for preventing the deposition of metal cations on a cathode being used for anodically passivating a vessel containing a corrosive electroplating bath.

Various electroplating processes have been used for many years and basically comprise an electroplating bath in which the member to be plated and a mass of the plating material are immersed, along with a current supply connected to the article to be plated and the mass in order to deposit metal cations from the mass onto the article to be plated. We have found that the vessel containing the electroplating bath may be protected from corrosion by an anodic passivation system. Such a system employs a cathode communicating with the bathand means for making the vessel an anode with respect to the cathode, whereby current is passed through the bath between the vessel and the cathode and corrosion of the vessel is minimized. However, the metal cations in the electroplating bath tend to migrate toward and deposit in relatively large amounts on the cathode of the anodic passivation system, thereby depleting the bath and resulting in prohibitive expense. We have further found, however, that deposition of the metal cations on the cathode may be prevented or reduced to a negligible amount by properly shielding the cathode from the electroplating bath.

The present invention features a shield between an electrode and an electrolytic solution containing ions which tend to migrate toward and deposit on the electrode, without interfering with the efiicient operation of the electrode in a system for controlling corrosion of the vessel containing the electrolytic solution. In the preferred embodiment, the shield takes the form of an inert tube inserted in the electrolytic solution, with a fritted glass bottom in the tube. A suitable metal-ion-free electrolyte (such as sulfuric acid) which will not affect the cathode is contained in the tube and the cathode is immersed in this electrolyte. The electrolytic solution and the electrolyte from the tube form a contacting interface within the fritted glass to provide electrochemical communication between the cathode and the plating bath. When current is passed between the cathode and the vessel being protected, hydrogen is liberated at the cathode, but no metal is deposited on the cathode. Current flow through the fritted glass is primarily by sulphuric acid anions, and only to a minor extent by cations from the plating bath.

An object of this invention is to increase the service United States Patent Ofice 3,437,572 Patented Apr. 8, 1969 life of electrodes used in lprotecting vessels containing corrosive electrolytic solutions having ions therein tending to migrate toward and deposit on the electrodes.

Another object of this invent-ion is to efficiently minimize the corrosion of the vessel of an electroplating system.

Another object of this invention is to anodically passivate the vessel of an electroplating system wherein the cathode of the anodic passivation system is shielded from metal cations in the electroplating bath.

A still further object of this invention is to provide a system for minimizing corrosion of a vessel containing substantially any corrosive electrolytic solution.

Other objects and advantages of the invention will be evident from the following detailed description when read in conjunction with the accompyaning drawing which illustrates the invention.

FIG. 1 is a schematic elevational view and wiring diagram of an electroplating system having an anodic passivation system installed thereon.

FIG. 2 is an enlarged schematic cross-sectional view of the shield for the cathode of the anodic passivation system shown in FIG. 1.

Referring to the drawing in detail, and particularly FIG. 1, reference character .10 designates a metal vessel which will normally be stainless steel in view of the corrosive environment to which the vessel is subjected. The vessel contains an electroplating bath 12 (a corrosive electrolytic solution), such as a metastable solution of nickel sulfate (one part), sodium hypophosphite (three parts) and small amounts caustic, tartaric acid, propionic acid and lactic acid, such as is used in the Kanigen process for electroplating nickel. The electroplating bath 12 may be at any necessary temperature for the electroplating operation, such as at a temperature of 206 F. (plus or minus 6), and at any necessary pH, such as 4.4 (plus or minus 0.1), as in the Kanigen process.

The article 14 to be plated (such as a metal tube) and a mass 16 of the plating metal (such as nickel) are suspended in the electroplating bath 12 in any suitable manner, such as by hooks 18 and 20* from suitable support rods 22 positioned above the top of the vessel 10. A DC power source 24 is suitably connected to the article 14 to be plated and the plating metal .16 in a direction to transfer metal from the mass 16 to the article 14. In the Kanigen process, the nickel 16 will be made the anode and the article .14 will be made the cathode as illustrated in the drawing.

In accordance with the present invention, a reference electrode 26 is placed in electrochemical communication with the electroplating bath 12. The reference electrode 26 may take any desired form, such as calomel electrode, a copper-copper sulphate electrode, or a silver-silver chloride electrode. Copper-copper sulphate and silver-silver chloride electrodes may be immersed directly in the plating bath. However, when using a calomel electrode 26, the electrode is connected to the electroplating bath .12 by means of a container 28 supported physically above the vessel 10 and containing a suitable electrolyte 30 such as potassium chloride. The container 28 is in turn connected to a weeping glass bridge 32 by a suitable tube 34 in order that the electrolyte 30 will gravitate downwardly into the weeping glass bridge 32. The weeping glass bridge 32 is suitably supported, as on the top 36 of the vessel 10, to extend downwardly into the electroplating bath 12. As is well understood by those skilled in the art, the weeping glass bridge 32 will provide a minute dripping of the electrolyte 30 into the electroplating bath 12 from the lower end of the bridge to provide electrical communication between the electroplating bath and the reference electrode 26, but an insufficient amount of the electrolyte 30 3 will seep into the electroplating bath to contaminate the bath.

The reference electrode 26 and the vessel are connected to a suitable controller 38 by conductors 40 and 42, whereby the controller 38 will monitor the difference in potential between the vessel 10 and reference electrode 26 and will operate a switch 44 in response to this difference in potential. Another DC power source 46 is provided for the anodic passivation of the vessel 10 and has its positive terminal 48 connected to the vessel 10 through a suitable conductor 50. The negative terminal 52 of the power source 46 is connected through the switch 44 to a cathode 54 shielded from the electroplating bath 12 by a shield 56.

The shield 56 is shown in detail in FIG. 2 and comprises a container in the form of an inert tube 58, such as a glass tube, inserted into the electroplating bath 12 and having its lower end 60 closed by a fritted glass plate or wall 62. The fritted glass plate 62 is suitably cemented or otherwise secured in the lower end of the tube 58 to prevent the electroplating bath 12 (and particularly metal cations in the bath) from coming into contact with the electrode 54. However, a metal-ion-free electrolyte 6 4, such as sulphuric acid, is placed in the tube 58 above the fritted glass plate 62 to provide electrical communication between the cathode S4 and the electroplating bath 12. The electroplating bath 12 and the electrolyte 64 will both partially migrate through the fritted glass plate 62 and form an interface in the body of the fritted glass to provide the electrical circuit referred to above.

The cathode 54 may be of any desired construction which is inert to the electrolyte 64 and may be, for example, a sheet of platinum.

The shield 56 is suspended in the electroplating bath 12 in any suitable manner, such as by a ring 66 positioned underneath a flange 68 on the upper end of the tube 58 and by a wire yoke 70 extending from the wire ring 66 over a support rod 72 fixed in a stationary position above the vessel 10.

OPERATION During operation of the electroplating system wherein metal cations are transferred from the mass 16 to the article 14 being plated, the electroplating bath 12 tends to corrode the inner surfaces of the vessel 10. Such corrosion varies the potential difference between the vessel 10 and the reference electrode 26 to operate the controller 38. When the potential reaches a predetermined level, the controller 38 closes the switch 44 and provides a passage of current between the vessel 10 and the cathode 54 to counteract or minimize the corrosive action. When the corrosion has been reduced to a predetermined minimum, this action is indicated by change in the potential difference between the vessel 10 and the reference electrode 26; whereupon the controller 38 opens the switch 44 to stop the flow of current between the vessel 10 and the cathode 54. This sequence is repeated as is necessary to retain the corrosion of the inner surfaces of the vessel 10 to a minimum, to not only increase the service life of the vessel but also minimize contamination of the electroplating bath 12.

When current is passed between the vessel 10 and the cathode 54 through the electroplating bath 12, free cations in the electroplating bath will tend to migrate away from the vessel 10 toward the cathode '54, and if the cathode is not shielded, these cations will deposit on the surface of the cathode 54 to seriously deplete the electroplating bath. In the present system, however, substantially the only ionic movement through the fritted glass plate 62 is by sulfuric acid anions. Hydrogen is liberated at the cathode '54, and only a small amount of metal cations pass through the fritted glass from the electroplating bath. Thus, the metal cations from the electroplating bath are prevented from depositing on the cathode 54 and the service life of the electroplating bath is materially extended.

To further show the effectiveness of the present system, an experiment was carried out using an apparatus of the type shown in FIG. 1, except for the mass 16 and member 14. The solution '12 contained 7.35 percent sulfuric acid plus one percent of stannous oxide which was soon converted to stannous sulfate. The vessel was 1020 mild steel and the solution was at F. 1.2 amperes of current were passed for one hour through the solution between the platinum cathode and the carbon steel vessel. No tin was deposited on the platinum cathode.

The cathode was then removed from the shield of pure sulfuric acid and placed directly in the sulfuric acid-tin plating solution. Then 1.2 amperes of current were passed for five minutes through the solution between the platinum cathode and the vessel surface. In the five minute interval, the ,4 inch thickness of the cathode sheet increased to about @7 of an inch due to the deposition of a layer of mossy tin.

From the foregoing it will be apparent that the present invention provides a novel system for minimizing the corrosion of a metal vessel containing corrosive electrolytic solution wherein ions from the solution tend to migrate toward an electrode suspended in the solution. The novel shield provided by this invention adequately prevents the deposition of ions, such as metal cations on the electrode used for controlling corrosion of the vessel. It will also be apparent that the present invention provides a novel system for anodically passivating the vessel of an electroplating bath wherein the cathode of the anodic passivation system is adequately protected from cations in the electroplating bath and the electroplating bath will have an extended service life.

Changes may be made in the combination and arrangement of parts or elements as heretofore set forth in the specification and as shown in the drawing, it being understood that changes may be made in the embodiment disclosed without departing from the spirit and scope of the invention as defined in the following claims.

We claim:

1. Apparatus for minimizing corrosion of a vessel member containing a corrosive electrolytic solution including free cations subject to being repelled by the vessel member when the vessel member is made an anode of a protective circuit, comprising:

a cathode member in electrical communication with the solution in the vessel member;

a DC circuit connecting the cathode member to the vessel member for passing current therebetween through the solution and anodically protecting the vessel member from corrosion; and

a shield between the cathode member and the solution preventing deposition of said cations from the solution onto the cathode member but at the same time permitting sufficient electrolytic communication to anodically protect said vessel.

2. Apparatus as defined in claim 1 wherein said shield comprises a tube of inert material having its lower end inserted in the solution, fritted glass secured across the lower end of the tube, and a chamber above said fritted glass for containing an electrolyte in the tube in contact with the electrode member.

3. Apparatus for minimizing corrosion of a metallic vessel containing a corrosive electroplating bath which in turn contains metal cations, comprising:

a reference electrode electrochemically communicating with the bath;

a current controller connected across the reference electrode and the vessel to monitor the difference in potential between the reference electrode and the vessel as the anode;

a cathode electrochemically communicating with the bath;

a source of DC connected to the cathode and the vessel as the anode to pass current through the bath and protect the vessel from corrosion;

means connecting the controller to the DC source to control the current passed through the bath between the cathode and the vessel in accordance with the diflference in potential between the reference electrode and the vessel; and

a shield between the cathode and the bath of such nature as to prevent the metal cations from depositing on the cathode when current is passed through the bath between the cathode and the vessel but at the same time permitting suflicient electrolytic communication to anodically protect said vessel.

4. Apparatus as defined in claim 3 wherein said shield comprises an inert container inserted in the bath having a chamber for containing:

a metal-ion-free electrolyte in the container in which the cathode is immersed; and

a porous membrane forming one wall of the container.

5. Apparatus as defined in claim 4 wherein said one wall is fritted glass.

6. In a process for anodically protecting a metallic vessel containing a corrosive electroplating solution and a cathode and wherein said vessel is the anode, the improvement which comprises inserting a shield, of such a nature as to prevent the passage of metal cations but at the same time permit sufficient electrolytic communication between said cathode and said metallic vessel to anodically protect said vessel, around said cathode.

References Cited UNITED STATES PATENTS 1,335,210 3/1920 VOn Wurstemberger 204196 2,684,938 7/1954 Mantzell 204195 2,954,336 9/1960 Grutsch 204195 3,030,280 4/1962 Miller 204195 3,032,493 5/1962 COulSOn et al. 204195 3,030,296 4/ 1962 McGlasson et al 204151 3,074,865 1/1963 Gaysowski 204196 3,126,328 3/1964 Hutchison et a1 204196 3,208,926 9/ 1965 Eckfeldt 204--195 3,288,694 11/1966 Banks 204196 JOHN H. MACK, Primary Examiner. T. TUNG, Assistant Examiner.

US. Cl. X.-R. 2041, 196, 231 

